1. Field of the Invention
The invention relates to headers for air-cooled heat exchangers.
2. Description of the Related Art
Air-cooled heat exchangers are items of equipment frequently used in industrial applications. Flowing a fluid, whether liquid or gas, through a series of tubes and drafting atmospheric air across the exterior of the tubes using one or more fans causes a heat transfer between the fluid and the atmosphere.
Air-cooled heat exchangers are commonly made for industrial applications by use of two spaced-apart headers. Tubes extend between the headers. The tubes are often finned and typically spaced closely together with one or more fans blowing atmospheric air over the tubes. The headers physically support and connect the tubes so that fluid will flow through all tubes. The headers with connected tubes and side frames which support a pair of opposed headers comprise an air-cooled exchanger section.
There are many types of headers, most of which have rectangular or round cross-section, and some of which have an oval cross-section. For example, Knulle (U.S. Pat. No. 4,130,398) discloses oval-shaped headers attached to double pipe elements.
Mosier (U.S. Pat. No. 3,689,972) discloses a pair of oval-shaped headers where the fluid tubes intersect the headers at highly curved portions of the headers.
Takeshita (U.S. Pat. No. 5,706,887) discloses a pair of headers of elliptic cross-section, connected by a single row of fluid tubes near the flatter portion of the ellipse. Other related patents are listed in the following table:
______________________________________ PATENT NO. INVENTOR TITLE ______________________________________ 1,929,365 Mautsch Heat Exchange Apparatus 3,689,972 Mosier et al. Method of Fabricating a Heat Exchanger 4,130,398 Knulle Oval Header Heat Exchanger and Method of Producing the Same 4,168,744 Knulle et al. Oval Header Heat Exchanger 5,036,914 Nishishita Vehicle-Loaded Parallel Flow Type Heat Exchanger 5,069,277 Nakamura Vehicle-Loaded Heat Exchanger of et al. Parallel Flow Type 5,076,354 Nishishita Multiflow Type Condenser for Car Air Conditioner 5,092,398 Nishishita Automotive Parallel Flow Type et al. Heat Exchanger 5,706,887 Takeshita Air Conditioner and Heat Exchanger et al. Used Therefor 5,727,626 Kato Header Tank of Heat Exchanger DE 2,500,827 Schmidt Double Tube Heat Exchanger Having Oval Collection Headers The Transfer Pieces Being Rolled Not Welded ______________________________________
There are problems with headers of existing art.
A rectangular header requires extensive welding to configure six flat steel plates into a rectangular box header. The four long seams at corner joints where the edges of the top, bottom, and side plates join together require welding as do the corner joints of the end plates where the two end plates join to the top, bottom, and side plates. In addition to the long length of these long seam welds, the depth of these welds increase with the thickness of the top and bottom plates to handle incremental pressure containment. These long corner joint welds result in significant fabrication time and expense. Hours of welding time and additional welding material are required for multiple weld passes to join the plates and fill the beveled joints. Additional expenses may incur for non-destructive testing on welded joints and possible rework of welded joints which fail non-destructive tests. Rework involves removal of welding material in the defective area, rewelding and re-testing.
There are other problems with headers of the existing art. Rectangular headers have right angle corners. Because the headers are under internal fluid pressure, there are stress concentrations acting at the right angle corners within the header walls. These stress concentrations contribute to potential failure of the header. Thus, it is generally preferred to have curved internal surfaces in pressure vessels.
Changing the header cross-section to a circle or oval partially solves this problem of large stress concentrations, but creates other problems. It is generally desired to have the tubes parallel to one another. Thus, when drilling openings in the header to receive the tubes, the drill bit must be maintained in a position normal to a diameter of the circular cross-section. Maintaining this angle makes effective drilling difficult at the top and bottom of a circular header, because the angle between the drill bit and the header surface becomes small. A related problem for headers of circular or oval cross-section is that it is more difficult to position and attach the tubes to a curved surface than it is to position and attach the tubes to a flat surface.
A further problem for curved cross-sections on the tube sheet portion of the header which is connected to the tubes is in the rolled connection of the tube to the header. The connection is typically made by inserting the tube into the tube hole and expanding the tube by the use of a rolling tool which is inserted into the interior of the portion of the tube within the thickness of the tube sheet portion of the header and expanding the outer circumference of the tube against the interior surface of the tube hole. It is undesirable to expand the tube beyond the outer wall of the tube sheet portion of the header since the tube may be weakened if this occurs. On curved tube sheet portions, the length of the tube which may be rolled is minimized as compared to headers with flat tube sheet portions.
Another problem for headers of circular or oval cross-sections is when the internal diameter of the tube holes in the tube sheet portion of the header are grooved for incremental tube-to-header securement. On curved cross-sections of the inner tube sheet wall, the area available for grooving is minimized since the grooves are positioned tangentially to the tube radius. As compared to flat tube sheet portions, the area available for grooving is minimized.
Another problem arises for headers of circular cross-section where the header has no access holes on the outer side of the header. Where there are access holes, the access holes are directly opposed to tubes which are connected to the tube sheet portion on the inner side of the header. Maintenance procedures typically utilize a straight rod to clean out the tubes. If the tubes are straight, the tubes can be cleaned with a mechanical cleaning device on the end of a straight rod. If the tubes are serpentine, the straight rods can clean out entrance and exit regions of the tubes, where solid particles tend to accumulate. If access holes do not exist, the tubes cannot be mechanically cleaned with external devices.
Another problem occurs for headers of circular or oval cross-section where threaded flat head shoulder plugs are used to plug access holes. The flat underside of the gasketed plug head does not fully engage with a curved surface. As a result for proper sealing, the access plug holes will require deep spot face machining to provide flat gasket surfaces on the curved plug sheet portion of the header which reduces the minimum thickness of the header wall resulting in additional material thickness in the header wall to contain a specified pressure. Tapered pipe thread plugs may also be utilized to seal access plug hole openings but utilization is limited to small diameter openings and containment of lower internal header pressures.
It is a primary object of this invention to minimize welding requirements.
It is another object of the invention to provide a header with minimal internal stress concentrations.
It is another object of the invention to provide a header with a minimum of right angle corners.
It is still another object of the invention to provide a header with a flat tube sheet portion on the inner side of the header for connection to fluid tubes.
It is yet another object of the invention to provide a header with access holes on a flat plug sheet portion to minimize required thickness of the header wall.
It is a further object of the invention to provide a header with threaded access plug holes on a flat surface which can be effectively sealed by plugs with either straight or tapered shanks.